The power efficiency of an amplification device such as a transistor varies with operating conditions. For example, a power amplifier designed for power efficiency at one supply voltage may well be inefficient at another supply voltage. However, applications abound that require the power amplifier to operate under varying conditions (e.g., different supply voltages) yet would benefit greatly from power efficient operation. In fact, while there are a variety of design constraints that may be imposed upon any power amplifier application, power efficiency is almost invariably a fundamental design objective.
By way of example, in most communication systems it is preferred that the power amplifier operate efficiently yet maintain an acceptable linearity over the desired supply voltage range. To that end, the typical power amplifier is adjusted to achieve a peak efficiency at a single output power level and supply voltage. In general, to operate at peak efficiency requires that the voltage swing at the output of the power amplifier be as large as possible. But, improving efficiency by increasing the voltage swing tends to reduce the linearity of the power amplifier. There is therefore a tradeoff between power efficiency and linearity, improvements in one coming at the expense of the other.
Many cellular communication systems, such as the CDMA cellular system, require that the power amplifier deliver a wide range of output powers. For more details regarding CDMA cellular systems, please see the Electronic Industry Association's publication EIA/TIA IS-95, which is incorporated herein by reference in its entirety. As will be appreciated, the power amplifier must safely operate at its highest power level. Being designed for the highest power level, the power amplifier tends to operate less efficiently at lower, more commonly used, power levels. Hence the life of a battery operated device is shortened because efficient power amplification is unavailable at the more commonly used power levels since the power amplifier must be designed for the highest power level.
Communication systems such as CDMA cellular systems merely illustrate one example of the multiplicity of power amplifier applications that require operation at numerous operating states. In order to address the shortcomings of the prior art, what is needed is a power amplifier that can achieve high power efficiency at several operating states.